1. Field of the Invention
The present invention relates to an optical pick-up apparatus for recording and/or reading information on and/or from an optical record medium such as optical disk.
There have been proposed various kinds of optical pick-up apparatuses for use in an optical recording and/or reproducing apparatus. In an optical pick-up apparatus of a so-called three beam type, one main beam and two sub-beams are made incident upon an optical record medium by means of a converging optical system including an objective lens, a focusing error signal is derived from the main beam reflected by the optical record medium by a beam size method and a tracking error signal is obtained from the two sub-beams reflected by the optical record medium.
In case of detecting the tracking error by the above mentioned three beam method, the main beam M is made incident upon a center of a track T on an optical record medium and two sub-beams S1 and S2 are made incident upon respective edges of the track T as shown in FIG. 1. When the optical record medium and the three beams are moved relative to each other in a tracking direction which is perpendicular to a track direction in which the information track T extends, a signal obtained from a photodetector receiving one of the sub-beams S1 changes as illustrated in FIG. 2A and a signal from a photodetector receiving the other sub-beam S2 changes as shown in FIG. 2B, so that a tracking error signal can be obtained by deriving a difference between these signals. These signals have opposite phases or a phase difference of these signals amounts to 180.degree., so that it is possible to obtain the tracking error signal having a large amplitude. During the adjustment or usage of the optical pick-up apparatus, the optical pick-up apparatus may be rotated about an optical axis which corresponds to a center of the main beam spot, so that the sub-beams might not be made incident upon the edges of the track T. Then, an amplitude of the output signals of the photodetector is reduced, and thus an amplitude of the tracking error signal is decreased. In this manner, the tracking error could not be detected at a high sensitivity. In order to mitigate such a drawback, it is desired to reduce a distance between the main beam spot and the sub-beams spots on the optical record medium in the track direction.
If the distance between the main beam spot and the sub-beam spots is shortened, the distance between the main beam spot and the sub-beam spots on the photo-detectors is also shortened, so that these beam spots interfere with each other. Then, not only the tracking error but also the focusing error and RF signal could not be detected correctly.
In Japanese Patent Application Laid-open Publication Kokai Sho 60-147941, there is described a solution for the above problem. In this solution, a concave lens is arranged in an optical path of the return beams reflected by the optical record medium and the distance between the main beam spot and the sub-beam spots on the photodetectors is increased. It is apparent that this solution requires the concave lens additionally, so that a cost and a size of the optical pick-up apparatus are increased.
In Japanese Patent Application Laid-open Publication Kokai Hei 4-248134 published on Sep. 3, 1992, there is proposed an optical pick-up apparatus of three beam type, in which the three return beams reflected by an optical record medium are diffracted by a hologram and .+-.1-order beams are received by photodetectors separately. In case of dividing the light beam by means of the hologram, a dividing angle cannot be large, so that it is very difficult to arrange the concave lens in optical paths of return beams between the hologram and the photodetectors. Therefore, the distance between the main beam spot and the sub-beam spot on the record medium could not be shortened sufficiently.
In order to avoid or mitigate the interference between the main beam spot and the sub-beam spots on the photodetectors, it is considered to increase a magnification of the converging optical system or to reduce a diameter of the three beam spots on the photodetectors is reduced. However, in the former solution, the optical path length becomes long and a size of the optical pick-up apparatus becomes large and in the latter solution a detectable range of the focusing error is limited.